Implantable medical devices (IMDS) which monitor cardiac signals are commonly used in monitoring and treating patients. For example, pacemakers can monitor cardiac activity and can also provide electrical stimulation to the heart. Some pacemakers stimulate the heart at regular intervals and others sense cardiac electrical activity and deliver electrical stimuli to the heart intermittently as needed. Therefore the ability to accurately detect and interpret cardiac electrical signals is important to cardiac monitoring and to the delivery of proper pacemaker therapy.
The ability of an IMD to sense cardiac electrical signals is typically controllable by means of circuitry for adjusting the sensitivity threshold of the pacemaker's sense amplifier, such that electrical signals resulting from depolarization of the cardiac muscle must exceed this sensitivity threshold in order for the cardiac event to be recognized. The sensitivity threshold may be defined in terms of a minimum voltage level for the input signal, or a minimum time period during which the input signal must exceed a minimum voltage level. The sensitivity threshold may be preset during manufacture or may be programmed by the physician at the time of implantation.
The sense amplifier circuitry of the IMD must be sensitive enough to ensure detection of cardiac signals, which are typically of relatively low magnitude, especially in the case of atrial sensing. However, the sense amplifier must not be so sensitive that certain non-electrical cardiac signals, such as electromagnetic noise, myopotentials, and the like, cause the IMD to erroneously sense a cardiac signal which did not actually occur. For example, in the case of pacemakers, if the sense amplifier circuitry is not sensitive enough (undersensing), the pacemaker could lose synchronization with the natural cardiac rhythm or deliver pacing stimuli at inappropriate times. However, if the sense amplifier circuitry sensitivity is set too low (oversensing), the pacemaker could erroneously sense a cardiac signal which did not occur.
The strength of cardiac signals received at the sense amplifiers inputs may change over time. Thus, a sensitivity threshold that is appropriate for a patient at the time of implantation might, at some later time, prove to be too high or too low, leading to undersensing or oversensing. Changes in the strength of the electrical cardiac signals received by the IMD may result from normal or pathological changes in the heart's intrinsic activity, from lead maturation effects such as changes in the positioning of implanted leads, or changes in the conductive properties of the heart muscle in the region surrounding the leads, such as might result from myocardial infarction and fibrotic tissue growth around the lead. Thus it is desirable for IMDs such as pacemakers to have an adjustable sensitivity threshold with the ability to determine an optimal sensitivity level.